Fluoropolymers, and in particular, vinylidene fluoride polymers, have been used with success in a wide variety of applications including electrochemical applications.
In particular, fluoropolymers are used as raw materials for polymer membranes for use in electrochemical devices such as secondary batteries because of their chemical and thermal aging resistance.
A metal ion secondary battery is typically formed by assembling a positive electrode (anode), a ion conducting membrane and a negative electrode (cathode); the ion conducting membrane, often referred to as separator, plays a crucial role in the cell, as it must provide ionic conductivity while ensuring effective separation between the opposite electrodes.
Basically, two types of separators can be used: either porous ones, wherein a solution of an electrolyte in a suitable solvent fills the porosity of the separator, or non-porous ones, which are generally either pure solid polymer electrolytes (i.e. electrolytes dissolved in a high molecular weight polyether host, like PEO and PPO, which acts as solid solvent) or gelled polymer electrolyte systems, which incorporate into a polymer matrix a plasticizer or solvent capable of forming a stable gel within the polymer host matrix and an electrolyte.
Nevertheless, gelled polymer electrolytes might not incorporate and retain the liquid plasticizer/electrolyte solution in an effective manner during both manufacturing of the battery and operations of the same, and/or might not possess suitable mechanical properties as required for effective separation of the electrodes.
On the other side, hybridization of organic and inorganic compounds is an important and evolutionary way to create polymeric compounds having, notably, enhanced mechanical properties. To elaborate such organic-inorganic polymer hybrids, sol-gel processes using metal alkoxides is the most useful and important approach. By properly controlling the reaction conditions of hydrolysis and polycondensation of metal alkoxydes, in particular of alkoxysilanes (e.g. tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)), in the presence of pre-formed organic polymers, it is possible to obtain hybrids with improved properties compared to the original compounds.
Within this scenario, WO 2011/121078 (SOLVAY SOLEXIS S.P.A.) Oct. 6, 2011 discloses certain fluoropolymer-based hybrid organic/inorganic composites wherein covalent bonds connect fluoropolymer chains to the inorganic domains, said composites being obtained by a process involving the reaction of certain functional fluoropolymers possessing hydroxyl groups with certain hydrolysable compounds of Si, Ti, or Zr, and subsequent hydrolysis and polycondensation of said compounds. This patent document also mentions that the so obtained hybrid organic/inorganic composites can be notably used for the manufacture of membranes for electrochemical applications, and more particularly as separators for lithium ion batteries. Thus, certain embodiments have been exemplified in such patent document, wherein films made of the mentioned hybrid organic/inorganic composites were swelled with an electrolyte solution comprising a solvent (mixture of ethylene carbonate and propylene carbonate) and an electrolyte (LiPF6). Nevertheless, once the film has been casted, swelling it again with an electrolyte solution is not an easy task, so that final amount of electrolyte solution actually interpenetrated in the separator is relatively low, so as, consequently, the ionic conductivity.
Ionic liquids are formed by the combination of cations and anions and are in the liquid state at temperatures close to ambient temperature. They have remarkable properties such as non-volatility, high ionic conductivity as well as catalytic properties. They are currently used in a wide variety of fields, in particular as electrolytes.
For instance, ion-conducting gels in solid form have been described in US 2012/0021279 (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE) Jan. 26, 2012, wherein they are obtained by mixing an ionic liquid with at least one sol-gel precursor containing at least one hydrolysable group. The ion-conducting gels so obtained are continuous films, are stable up to temperatures of approximately 250° C., are transparent, are electrical insulators and are ionic conductors.
Although these ion-conducting gels have many useful properties, their use as electrolytes has many drawbacks due to their low mechanical strength.